Influence of calcination temperature of impure kaolinitic clay on hydration and strength development of ultra-high-performance cementitious composite

Abstract

Despite the recent considerable interest in sustainable supplementary cementitious materials, the effect of calcined impure clay on ultra-high-performance cementitious composite (UHPC) is insufficiently understood. In this study, three impure kaolinitic clays calcined at 600 °C, 800 °C, and 1000 °C were compared with quartz powder in terms of hydration reaction and strength development of UHPC. As a function of calcination temperature, the clays had different effects on the properties of UHPC. The clays calcined at 600–800 °C increased the initial strength of UHPC (≤3 days) by ∼10–35% but show lower strength by ∼5–9% after 28 days compared to reference sample. It is because the clays accelerated both cement hydration and pozzolanic reaction at an early age of the UHPC, but the premature consumption of water and pore refinement inhibited the long-term pozzolanic reaction. In contrast, the clay calcined at 1000 °C no longer affected the UHPC’s strength after increasing the 1st-day strength by 17%, since it acted as an inert filler after early age (>1 day). However, once heat treatment (at 90 °C for 48 h) was applied to UHPC, all of the clays did not express pozzolanic reactivity because the main initiative for the pozzolanic reaction shifted from metakaolin to silica fume. From these results, it is concluded that the use of calcined clays does not notably reduce the strength of UHPC, as long as impure clays with sufficient kaolinite content (>40%) are used.